It is the current practice in the field of internal combustion engines to provide a good fuel economy by interrupting the fuel delivery to the engine in response to an operator's demand for deceleration. During the fuelcut control, however, the whole amount of air introduced into the engine is discharged into the exhaust system to increase the amount of oxygen supplied to the catalytic converter. As a result, oxidation is promoted rapidly to increase the catalyst temperature so as to degrade the catalysts.
For example, Japanese Patent Kokai No. 2-91438 discloses an air/fuel ratio leaning control made during deceleration to provide good fuel economy without excessive catalyst temperature increase. If a fuelcut control is made to produce a leaned air/fuel ratio during deceleration at high engine-speed and high engine-load conditions increasing the catalyst temperature, however, an excessive amount of hot air will enter the catalytic converter where the oxygen included in the hot air is jointed to the rhodium (Rh) contained in the catalysts. This results in a temporary reduction of the pollutant purifying capacity of the catalytic converter.